Spark plug with multiple conductive seals



United States Patent 3,274,423 SPARK PLUG WITH MULTIPLE CONDUCTIVE SEALSPatrick N. Kesten, Davison, Mich, assignor to General MotorsCorporation, Detroit, Mich, a corporation of Delaware Filed Mar. 5,1962, der. No. 177,423 4 tllaims. (Cl. 313145) This invention relates tospark plugs and, more particularly, to a spark plug of the type having asilver center electrode and to a method for making same.

In recent years it has become the practice to use a solid silver centerelectrode in certain types of spark plugs, particularly those for use intwo-cycle engines such as outboard marine engines, power lawn mowerengines and the like. Such an electrode has the advantage of extremelyhigh heat-conductivity and has been found to display exceptional wearcharacteristics. In the manufacture of such spark plugs, after thesilver center electrode is inserted'into the insulator it is necessaryto hermetically seal the insulator against the egress of exhaust gasesduring spark plug operation. This is accomplished by inserting aconductive'sealing material over the electrode and then applyingpressure to the material such that it is pressed into sealing engagementwith the insulator wall. In practice it has been found that qualitycontrol of such spark plugs is extremely difiicult for the reasons thatthe relatively soft silver electrode tends to extrude out of theinsulator while the pressure is applied. This unwanted extrusion causesthe electrode firing tip to extend too far from the end of the insulatorand it also frequently results in poor electrical contact between thetop of the electrode and the conductive sealing material.

It is an object of the present invention to provide an improved silverelectrode type spark plug which solves the aforementioned problem.Another object of the invention is the provision of an improved methodfor manufacturing such spark plugs which assures uniform qualitycontrol. Briefly, these objects are accomplished in accordance with theinvention by inserting a charge of metal powder between the top of thesilver electrode and the seal. The metal powder has been found tominimize extrusion of the silver electrode while pressure is applied inthe sealing operation. Hence, excellent quality control is attained bothas regards the electrode firing tip length and as regards the electricalcontact between the electrode and the seal.

Other objects, features and advantages of the invention will appear moreclearly from the following detailed description thereof made withreference to the accompanying drawing which shows a side view in sectionof a preferred embodiment.

Referring now to the drawing, the spark plug comprises a conventional,generally tubular steel shell 2 with ground electrode 3 welded to thebottom thereof and a ceramic insulator a secured concentrically withinthe shell in the conventional manner. The insulator has a centerborewith a straight cylindrical lower portion 5 of relatively small diameterand an upper cylindrical portion 6 of relatively large diameter, aninternal annular shoulder 7 constituting the junction of these diiferentsized bore portions. The conductive path through the insulatorcenterbore includes a solid silver center electrode 8 which ispositioned in the small diametered centerbore portion 5 and a terminalwire 9 which extends into the large bore portion 6. A head It) which isintegral with the terminal wire 9 and abuts the top of the insulatorserves as the electrical contact for the spark plug with the ignitionsystem. The lower end portion of the wire 9 is embedded in a fusedconductive glass seal 11 which can 3,274,423 Patented Sept. 20, 1966consist of a mixture of about 45% borosili-cate glass and 55% nickel orcopper powder, such sealing mixtures being well known and described inprior patents such, for example, as United States Patent 2,248,415. Inorder to provide increased surface contact with the conductive glassseal the lower end of the wire 9 is preferably threaded as shown. Theupper end of the silver electrode 8 is shaped with an enlargement in theform of a flange 12 which rests against the insulator shoulder 7. Theportion of the electrode above the flange extends into the largecenterbore portion 6 and is shaped with a plurality of circumferentiallyspaced, outwardly extending ribs 13 to increase its surface area. Inaccordance with the invention, a body of tightly compressed metal powder14 extends between the conductive glass seal and the silver electrodeand encircles the upper ribbed portion of the latter. In the embodimentshown, the powder 14 consists of nickel having a mixed grain sizeranging from minus 325 mesh to minus 48 mesh. The use of mixed particlesof the designated size range permits maximum filling around and betweenthe ribs 13 in a close-packed relationship reducing voids which isbelieved to be a factor in minimizing extrusion. The powder also tendsto distribute the sealing pressure and provides increased lateralsupport to the electrode due to the larger surface area of the ribs 13as compared to the surface area of the electrode upper end. It will beunderstood, however, that other metal powders may be used within thepurview of the invention.

In the manufacture of the spark plug, the silver electrode is insertedinto the insulator centerbore from the top thereof until it rests in theposition shown. Then a charge of nickel powder is poured into thecenterbore against the top of the silver electrode. This is followed bythe addition of a charge of glass-metal powder mixture after which thewire 10 is inserted. At this stage the bottom of the wire 9 will restagainst the top of the glass seal charge and hence the head 10 will bespaced upwardly from the top of the insulator. Intense heat is thenapplied until the glass sealing material becomes molten, and downwardpressure is applied to the wire 9 thereby compressing the metal powder14 and causing the threaded end of the wire 9 to sink into the moltenglass sealing material. Upon cooling at the conclusion of this operationthe glass sealing material is bonded to the insulator and to the wire 9thereby creating a hermetic seal.

As has been mentioned above, the big advantage to the metal powder layeris that it minimizes the tendency of the relatively soft silverelectrode from extruding downwardly out of the centerbore and away fromits flange 12 during the application of pressure in the glass sealingoperation. Hence, there is assurance of proper spacing between thebottom of the center electrode and the ground electrode 3 upon finalassembly of the plug, and there is also assurance of excellentelectrical contact, through the metal powder, between the conductiveglass seal and the top of the center electrode.

While in the preferred embodiment described, the seal comprises a fusedconductive mixture of glass and metal powder, it will be understood thatthe invention also has utility in spark plugs wherein other types ofpressed centerbore seals are used. For example, the centerbore seal canconsist of a tightly pressed mixture of ceramic and metal powder or, asis sometimes used, a body of pressed fused glass with a thin metal wireextending therethrough to form the electrical connection between theterminal and the center electrode. These and other centerbore sealswhich require the application of pressure thereto during manufacturecreate the same problem as regards the use of a silver electrode, i.e.,extrusion of the electrode during manufacture. Hence, the presentinvention wherein metal powder is interposed between the centerelectrode and the seal will serve to advantage in all such spark plugs.It will be understood, therefore, that while the invention has beendescribed specifically with reference to a preferred embodiment thereof,various modifications may be made, all within the full and intendedscope of the claims which follow.

I claim:

1. A spark plug comprising an insulator having a centerboretherethrough, an elongated silver electrode in said centerbore having afiring tip portion at the lower end thereof and a ribbed portion at theupper end thereof, a pressed seal in said centerbore above said silverelectrode and a mass of tightly packed metal powder in said centerborebelow said seal surrounding said ribbed portion, said mass minimizingthe tendency of said silver electrode to extrude during the manufactureof said plug.

2. A spark plug comprising an insulator having a centerboretherethrough, said centerbore having a lower portion of relatively smalldiameter, an upper portion of enlarged diameter and an annular shoulderat the junction of said portions, an elongated silver electrode in thelower portion of said 'centerbore having a firing tip portion at thelower end thereof and an enlargement at the upper end thereof positionedin said upper centerbore portion and abutting said annular shoulder,said silver electrode having a plurality of ribs above said enlargementa pressed seal in the upper portion of said centerbore and a mass oftightly packed metal powder in said centerbore below said sealsurrounding said ribs, said mass minimizing the tendency of said silverelectrode to extrude during the manufacture of said plug of said silverelectrode.

3. A spark plug as defined in claim 2 wherein the seal comprises a fusedmixture of glass and metal powder.

4. A spark plug as defined in claim 2 wherein the packed metal powder isnickel having a mixed size range of from minus 325 to minus 48 mesh.

References Cited by the Examiner UNITED STATES PATENTS 2,478,087 8/1949Bychinsky et al. 313-136 X 2,487,531 11/1949 Dutterer 3l3136 X 2,615,44110/1952 Bychinsky 313--l36 X 2,837,679 6/1958 Schwartzwalder et al.

313136 X FOREIGN PATENTS 836,165 6/1960 Great Britain.

JAMES W. LAWRENCE, Primary Examiner.

ARTHUR GAUSS, GEORGE N. WESTBY, Examiners.

C. R. CAMPBELL, Assistant Examiner.

2. A SPARK PLUG COMPRISING AN INSULATOR HAVING A CENTERBORETHERETHROUGH, SAID CENTERBORE HAVING A LOWER PORTION OF RELATIVELY SMALLDIAMETER, AN UPPER PORTION OF ENLARGED DIAMETER AND AN ANNULAR SHOULDERAT THE JUNCTION OF SAID PORTIONS, AN ELONGATED SILVER ELECTRODE IN THELOWER PORTION OF SAID CENTERBORE HAVING A FIRING TIP PORTION AT THELOWER END THEREOF AND AND ENLARGEMENT AT THE UPPER END THEREOFPOSITIONED IN SAID UPPER CENTERBORE PORTION AND ABUTTING SAID ANNULARSHOULDER, SAID SILVER ELECTRODE HAVING A PLURALITY OF RIBS ABOVE SAIDENLARGEMENT A PRESSED SEAL IN THE UPPER PORTION OF SAID CENTERBORE AND AMASS OF TIGHTLY PACKED METAL POWDER IN SAID CENTERBORE BELOW SAID SEALSURROUNDING SAID RIBS, SAID MASS MINIMIZING THE TENDENCY OF SAID SILVERELECTRODE TO EXTRUDE DURING THE MANUFACTURE OF SIAD PLUG OF SAID SILVERELECTRODE.